Method and trancvier for digital subscriber line system

ABSTRACT

This disclosure provides a method and transceiver for a digital subscriber line (DSL) system. A first transceiver performs a communication with a second transceiver with a bit table including a first parameter of bit load for sub-carriers, and transmits a switch message for requesting a bit load reduction to the second transceiver when the first transceiver determines that an obtained communication performance parameter reaches or exceeds the predetermined value; and in response to the transmission of the switch message, executing a switch to a second parameter of bit load for sub-carriers at a certain symbol of a frame. The first transceiver calculates the second parameter of bit load by subtracting a corresponding bit number from the first parameter of bit load. The corresponding bit number is transmitted by the first transceiver to the second transceiver for calculating a same parameter of bit load as the second parameter of bit load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of US patent application Ser. No. 13/491,234,filed on 7 Jun. 2012, and title “method and transceiver for digitalsubscriber line system,” which is a continuation of U.S. patentapplication Ser. No. 12/249,659, filed on Oct. 10, 2008 and now issuedas U.S. Pat. No. 8,213,488, which is a continuation of PCT applicationNo. PCT/CN2007/000196, filed on Jan. 18, 2007, and which claims thepriority of Chinese patent application No. 200610072439.0, filed on Apr.13, 2006, the entire contents of all of which are incorporated herein byreference.

FIELD

The present embodiments relate to a communication technique, especiallyto a method and a device for performing communication in digitalsubscriber line technology.

BACKGROUND

After several years of development, ADSL (asymmetric digital subscriberline) technology has developed from the first generation to ADSL2(second generation ADSL), ADSL2+ (ADSL2 with downlink bandwidth beingextended) and more recent VDSL2 (second generation very high ratedigital subscriber line). The frequency band being used is increasinggradually, and the bandwidth is also increasing gradually. The ADSL andADSL2 employ a frequency spectrum below 1.1 MHz in downlink and are ableto provide a maximum downlink rate of 8 Mbps, the ADSL2+ extends thedownlink bandwidth to 2.2 MHz and is able to provide a maximum downlinkrate of 24 Mbps, and the VDSL2 can even employ a frequency spectrum upto 30 MHz and is able to provide a maximum access rate of 100 Mbit/swhich is symmetrical in uplink and downlink. The above digitalsubscriber line technologies are totally called as xDSL.

Because the transmission medium for the xDSL is an unshieldedtwisted-pair, and electromagnetic coupling exists between differenttwisted-pairs, the signal transmitted on a twisted-pair may betransmitted to another twisted-pair through electromagnetic coupling toform a crosstalk. To reduce such a crosstalk, the twisted-pairs adoptdifferent pitches, and the xDSL adopts differential signal transmissionand reception, to counteract the common mode interference signal as faras possible by using the symmetry of twisted-pairs. However, in reality,the symmetry of twisted-pairs is relative and the crosstalk stillexists. In addition, the interference signal in the ambient environmentmay also be coupled to the twisted-pairs, because the symmetry oftwisted-pairs is limited and the interference signal can be convertedinto a differential mode signal to cause interference.

The crosstalk between pairs may greatly affect the service. For example,when a pair 1 is trained, its adjacent lines have no service, and ahigher activate rate can be achieved with respect to a given signal tonoise ratio margin. Hereafter, the adjacent lines also start totraining, and signals emitted from these lines cause a crosstalk signalon the pair 1, which may result in a noise increase that may reach toten and more dBs. At this time, the originally set signal to noise ratiomargin of the pair 1 (generally of 6 dB) cannot ensure the operation atthe original bit error rate and rate by the lines. At this time, thebetter case may result in an increased bit error rate, and the worsecase may result in link break and re-training, causing a serviceinterruption. This problem can be more serious in case of VDSL2. Becausethe VDSL2 means a higher frequency and a shorter line, and the remotecrosstalk increases with the frequency and decreases with the increaseof distance, the crosstalk has a greater influence. The ADSL2+ defines afast training mode. Although the fast re-training may recover theconnection within a minimum duration of 3 seconds, an influence to theservice cannot be completely avoided. Moreover, some services such asvoice over IP may require to re-connect due to problems such as linkdrop, and therefore, keeping a good communication connection quality(for example, no link drop) may be very important to service quality anduser experience.

In the prior art, there are three kinds of solutions to solve thecrosstalk caused by changing the adjacent line from unusable still state(in the silent state, there is no signal in the line) to normal use. Theabove three kinds of technical solutions will be respectively describedin the following.

Solution one, by increasing the target Signal to Noise Ratio (SNR)margin of the pair 1, a larger signal to noise ratio margin is reservedwhen training the pair 1, so that when the crosstalk suddenly increases,the communication can still keep the target bit error rate as long asthe increase does not reach to or exceed the target signal to noiseratio margin, and there is an enough margin to avoid the re-training.This solution has the benefits of simplicity and practicability, butalso the deficiency that increasing the signal to noise ratio margin maya reduced rate that can be achieved on the pair 1. Moreover, because thecrosstalk noise is generally not flat, that is to say, noise powerspectrum densities at different frequency points are different, and thesignal to noise ratio margin is a flat value such that substantiallyequal values are reserved as margins for signal to noise ratios of allthe sub-channels, in view of a fact that the crosstalk is serious onlyin some frequency ranges, a too higher signal to noise ratio will wastethe transmission capacity in the frequency bands where the crosstalkinfluence is very small.

Solution two is a seamless rate adaptation (SRA) solution. When thesignal to noise ratio of the line reduces due to the crosstalk, the SRAsolution ensure the signal to noise ratio margin by reducing the numberof bits modulated on the sub-carriers being affected, so that the biterror rate is not higher that the target value. According to thissolution the bit allocating can be adjusted automatically according tothe noise distribution, so as to avoid the problem of solution one.However, because it is necessary in the SRA solution to calculate andupdate bit tables and gain tables for the sub-carriers (bit allocationtables and gain adjustment tables for the sub-carriers in case ofmultiple carrier communication. See ADSL or VDSL standard of ITU-T), theamount of the data is very large. Limited by transmission capability ofthe channel for transmitting the overhead, the solution has a lowerresponse speed. However, the crosstalk from the adjacent lines suddenlyincreases at the moment of entering the training, and therefore there-training may be performed due to consecutive failures, beforecompleting the adjustment to the transceiver. Further, it is necessaryin the SRA solution to transmit a lot of data (bit and gain table)between the receiving device and the transmitting device. However, thesignal to noise ratio of the channel has reduced, and the process ofupdating the bit and gain tables may fail due to errors.

Solution three, in the ITU-T G993.2 standard (also called VDSL2), aconcept of virtual noise (VN) is introduced, which is a noise obtainedby shaping as required. FIG. 1 shows a relation between the virtualnoise and the actual noise, where the dashed line represents a virtualnoise variation curve and the solid line represents a noise variationcurve. If such a virtual noise is used to calculate the signal to noiseratio and the bit load of each of the sub-carriers is calculated, a VNbased line rate can be obtained. By setting an appropriate VN not lowerthan the maximal possible crosstalk noise in a basic bundled unit of thecable (for example, in case of VDSL, for a basic unit of 25 pairs, theVN is setting as not lower than the crosstalk generated when 24 pairs oflines are activated at the same time), the pair 1 will not suffer are-training even if these pairs are trained after the pair 1 reaches theshowtime (a term for special use in the standard, also called operatingstate)Moreover, because of adopting a shaped noise, enough margins areonly reserved on the required sub-channels to avoiding the waste due tosimply setting a flat target signal to noise ratio margin. However, thissolution is still a conservative solution because of the followingreason. For security, it is required to design the VN according to themaximal possible crosstalk noise, for example, the worst case of 1%.However, the crosstalk is not so bad actually in many cases, or is inthe worst case only during a very short period, so that the solutionalways running in this conservative mode can still making the waste ofchannel capacity.

SUMMARY

The embodiments provide a method and system for performing communicationin digital subscriber line technology. The transmission rate may beadapted according to the noise change in the line, the anti-noiseability may be improved in order to avoid various problems (such as,link drop) caused by the large increase of line noise. The method andsystem are especially applicable to the case that the noise suddenly hasa large increase in a short time. When the line noise is reduced, theline rate may be dynamically increased, thereby improving thetransmission capacity.

A method of performing communication in digital subscriber linetechnology, including: acquiring a communication performance parameter;and using, by the first transceiver and the second transceiver, apredetermined communication rule to perform communication when thecommunication performance parameter reaches or exceeds a predeterminedvalue.

A transceiver in digital subscriber line technology, including: areceiving module configured to receive a signal from a subscriber line;a monitor module configured to acquire a communication performanceparameter based on the received signal from the receiving module; and aprocessing module configured to perform a switch according to apredetermined communication rule based on the line quality parameterfrom the monitor module.

A transceiver system in the digital subscriber line technology,including: a first transceiver and a second transceiver forcommunicating through a subscriber line.

The first transceiver obtains a communication performance parameter, andthe first transceiver and the second transceiver use a predeterminedcommunication rule to perform communication when the communicationperformance parameter reaches or exceeds a predetermined value.

According to the embodiments, a bit table and a gain table aredetermined in advance and the determined bit table and gain table arerespectively saved in the first transceiver and the second transceiver,or a calculation rule that can be understood and used by the firsttransceiver and the second transceiver is determined in advance. When alarge wideband noise (for example, crosstalk) presents, a fast switchfrom the currently used bit table and gain table to the bit table andgain table determined in advance is performed by using a simple messageor query-response mechanism, or a new bit table and gain table arecalculated by using the currently used bit table and gain tableaccording to the rule determined in advance and a fast switch to the newbit table and gain table is performed. Because it is not necessary toexchange the bit table and gain table between the first transceiver andthe second transceiver, the solutions according to the embodiments havethe advantages of fast switch speed and high reliability. When the abovewideband noise decreases (for example, the user of a crosstalk sourceturns off the modem), the SNRM may be calculated according to thecurrent channel, and the frequency spectrum utilization may be increasedby increasing the rate through the SRA.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example about the relation between the virtual noise theactual noise;

FIG. 2 shows a schematic diagram of a receiving system according to oneembodiment;

FIG. 3 shows a schematic diagram of the relation between the line noise,the signal to noise ratio margin and the line rate according to oneembodiment;

FIG. 4 shows a schematic diagram of a query message and a responsemessage according to one embodiment;

FIG. 5 shows a query-response process between the first transceiver andthe second transceiver according to one embodiment;

FIG. 6 is a schematic diagram showing the case where a failure occurs inthe query-response process between the first transceiver and the secondtransceiver according to one embodiment; and

FIG. 7 shows a schematic diagram of performing a bit table switch byusing the query-response process between the first transceiver and thesecond transceiver according to one embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown in FIG. 2, one embodiment discloses a transceiver system in thedigital subscriber line technology, including: a first transceiverconfigured to acquire a communication performance parameter; and asecond transceiver configured to acquire a communication performanceparameter. If the communication performance parameter reaches or exceedsa predetermined value, the first transceiver communicates with a secondtransceiver according to a predetermined communication rule which isprovided in the first transceiver, otherwise based on the parameters ina bit table and a gain table calculated according to the actual noise.If the communication performance parameter reaches or exceeds apredetermined value, the second transceiver communicates with the firsttransceiver according to a predetermined communication rule which isprovided in the second transceiver, otherwise based on the parameters ina bit table and a gain table calculated according to the actual noise.

The configuration and the function of the first transceiver are the sameas that of the second transceiver. One of them may be arranged at theoffice side, and another of them may be arranged at the subscriber side.The transceivers includes: a receiving module configured to receive asignal and a message from the peer transceiver; a monitor moduleconfigured to obtain a communication performance parameter from theacquiring, the communication performance parameter including signal tonoise ratio, signal to noise ratio margin and bit error rate; aprocessing module configured to perform the switch of the predeterminedcommunication rule according to the communication performance parameterfrom the monitor module and generate a switch message, and perform theswitch for the predetermined communication rule according to the switchmessage, the predetermined communication rule including the bit tableand/or the gain table; a transmitting module configured to transmit asignal and the switch message generated by the processing module to thepeer transceiver.

The processing module includes: a storage unit configured to store thebit tables and the gain tables respectively according to the twocommunication rules and the bit tables and the gain tables calculatedaccording to the actual noise, and some configuration parameters such asswitch threshold for triggering the bit tables and the gain tables (bi &gi) defined according to the bit error rate or the signal to noise ratiomargin; a judgment unit configured to judge whether the communicationperformance parameter reaches or exceeds the predetermined value, and ifthe communication performance parameter reaches or exceeds thepredetermined value, generate a switch message and transmitting theswitch message to the peer end through the transmitting module, wait fora synchronization signal from the peer end, and initiate a switch unitif the synchronization signal is obtained from the peer end within apredetermined time; the switch unit configured to switch to thepredetermined bit table and the predetermined gain table.

The transceivers also includes: a seamless rate adaptor configured tocalculate a bit table and a gain table according to the actual noise,and take them as a new communication rule to increase the rate when thenoise decreases. The receiving module is also configured to receive thebit table and the gain table transmitted from the peer end and thetransmitting module is also configured to transmit the bit table and thegain table to the peer end.

The embodiments also disclose a method of performing communication inthe digital subscriber line technology, including: obtaining acommunication performance parameter; judging whether the communicationperformance parameter reaches or exceeds the predetermined value, if thecommunication performance parameter reaches or exceeds the predeterminedvalue, notifying the peer transceiver through a message and making thetransceivers of both ends to using parameters in the predetermined bittable and the predetermined gain table to perform the communication, andif the communication performance parameter does not reach or exceed thepredetermined value, making them to use parameters in the bit table andthe gain table calculated according to the actual noise to perform thecommunication.

To obtain the communication performance parameters, the monitor modulecontinues to monitor the signal received by the receiving module, andobtains the communication performance parameters such as signal to noiseratio, signal to noise ratio margin and bit error rate from the receivedsignal, so as to notify the peer transceiver when one or more of theseparameters reach or exceed predetermined values. Therefore, thetransceivers of both ends adopt parameters in the predetermined bittable and the predetermined gain table to perform the communication.

Upon obtaining the communication performance parameter, the judgmentmodule judges whether the communication performance parameter reaches orexceeds the predetermined value. If the communication performanceparameter reaches or exceeds the predetermined value, the peertransceiver is notified through a message and the transceivers of bothends are instructed to use parameters in the predetermined bit table andthe predetermined gain table to perform the communication. If thecommunication performance parameter does not reach or exceed thepredetermined value, the transceivers of both ends are instructed to useparameters in the bit table and the gain table calculated according tothe actual noise to perform the communication.

To judge whether the communication performance parameter reaches orexceeds the predetermined value, a virtual noise or experiential bittable calculating parameter, a signal to noise ratio margin thresholdand/or a bit error rate threshold may be pre-set in the firsttransceiver (assuming that the first transceiver is arranged at theoffice side) before the training. The experiential bit table calculatingparameter is a bit number reduced from the bit number calculatedaccording to the actual noise to provide the bit number of the signal tonoise ratio margin on each tone (sub-frequency band), then the virtualnoise or experiential bit table calculating parameter, the signal tonoise ratio margin threshold and/or the bit error rate threshold as setare saved in the storage module of the first transceiver. In order tosend the virtual noise or experiential bit table calculating parameter,the signal to noise ratio margin threshold and/or the bit error ratethreshold to the second transceiver (assuming that the secondtransceiver are arranged at the subscriber side) through informationinteraction in the training process, and the second transceiver savesthe virtual noise or experiential bit table calculating parameter, thesignal to noise ratio margin threshold and/or the bit error ratethreshold in the storage module. In the training, the processing moduleof the second transceiver calculates two sets of signal to noise ratiotable, bit table and gain table respectively based on the preset virtualnoise or experiential bit table calculating parameter and the actualnoise: SNRvi, Bvi and Gvi, and SNRri, bri and Gri, and saves the SNRvi,Bvi and Gvi, and the SNRri, bri and Gri in the local storage module andtransmits them to the first transceiver through information interaction,in order to save them in the storage module of the first transceiver.Then the first transceiver and the second transceiver implement theconnection based on the SNRri, bri and Gri calculated according to theactual noise.

The process of judging whether the communication performance parameterreaches or exceeds the predetermined value is described as follows. Thefirst transceiver and/or the second transceiver compare the monitoredsignal to noise ratio, signal to noise ratio margin and bit error ratewith the preset thresholds stored in their storage modules. Taking themonitoring of the signal to noise ratio margin as an example, when it isfound that the signal to noise ratio margins of multiple sub-carriers(according to the power distribution of the crosstalk, there are manyconsecutive sub-carriers in most cases), for example, 10 sub-carriers,are lower than the preset thresholds, which indicates a case where thetraining of an adjacent pair causes a sudden increase of the crosstalk,the transceiver generates a switch message and transmits the switchmessage to the peer transceiver. Upon receiving the switch message, thepeer transceiver switches to the bit table and the gain table calculatedbased on the virtual noise or experiential bit table calculatingparameter, and then returns a confirmation message and a synchronizationmessage (it is also possible to only return a synchronization message).Upon receiving the confirmation message and the synchronization message,the transceiver switches to the bit table and the gain table calculatedbased on the virtual noise. Thus, the first transceiver and the secondtransceiver can perform the communication by using the bit table and thegain table calculated from the virtual noise.

The predetermined bit table and the predetermined gain table may beobtained according to the preset virtual noise or may be an experientialbit table.

The virtual noise may be the noise in case of the worst line crosstalk.Thus, in the training, in addition to obtaining a signal to noise ratiotable SNRri, a bit table Bri and a gain table Gri according to theactual noise on the U-interface (an interface between the xDSLtransceiver and the external twisted-pair), the first transceiver andthe second transceiver also calculate another corresponding data (SNRvi,Bvi and Gvi) according to the preset virtual noise or experiential bittable calculating parameter. The signal to noise ratio table SNRri, thebit table Bri and the gain table Gri respectively represent the signalto noise ratio, the number of carried bits and the relative gainadjustment coefficient of each sub-carrier. (SNRvi, Bvi and Gvi) arerespectively saved in the storage modules of the first transceiver andthe second transceiver. When the transceiver detects that the signal tonoise ratio SNRi and/or the signal to noise ratio margin SNRMi and/orthe bit error rate BER reach or exceed a preset value due to thecrosstalk generated by training the adjacent pair, the transceivertransmits a switch message to the peer end transceiver, so that the peerend transceiver switches to the Bvi and the Gvi calculated according tothe saved virtual noise or experiential bit table calculating parameter,and the synchronous switch between the first transceiver and the secondtransceiver is implemented through the switch synchronization signal.

The experiential bit table may be obtained by experience, for example,according to experiential distribution of the crosstalk. By subtractinga corresponding bit number from the actual noise-based bit gain tableBri & Gri obtained according to the training result (the increase ofsignal to noise ratio margin caused by the subtracted bit number shouldbe greater than or equal to the decrease of signal to noise ratio margincaused by the crosstalk), it is necessary to design the virtual noiseand transmit it the peer end, thereby further simplifying theoperations.

Because the bit load and gain adjustment table calculated according tothe virtual noise is a very conservative solution, although the signalto noise ratio margin and the bit error rate meet the requirement(usually reach or exceed the requirement), the resultant rate is lowerthan the value in case of actual noise. The line rate may be increasedstep-by-step according to the actual SNRri. Accordingly, a fastdecreasing rate may be obtained to accommodate the sudden increase ofnoise, and to slowly increase the rate to accommodate the dynamic rateadjustment process with the noise being reduced. Because the lineparameters in this fast switch process are calculated in advance, andsaved in the first transceiver and the second transceiver, the switchcan be implemented only through a simple message and a synchronizationmessage when it is necessary to perform the switch. This is much fasterand more reliable than the original SRA or BS, because the SRA or BSneeds to transmit the updated bit table and gain table. In case of SRAor BS, a failure may occur in transmitting the bit table and the gaintable due to the bit errors in the transmission process, if the channelis poor. The solution according to the embodiment will not cause thisproblem, and when the line crosstalk noise decreases, the signal tonoise ratio increases and the bit error rate decreases, so that a largeamount of data required by the SRA can be transmitted.

When entering the showtime, the signal to noise ratio table SNRri, thebit table Bri and the gain table Gri calculated according to the actualnoise may act as transmission parameters, to achieve a highertransmission rate.

FIG. 3 shows a diagram of the relation between the line noise, thesignal to noise ratio margin and the line rate. As shown in FIG. 3, theabscissa axis represents time, and the relation between the line noise,the signal to noise ratio margin and the line rate are shown by theordinate. At time a, because the adjacent pair starts the training togreatly increase the line noise and sharply reduce the SNRM, accordingto the embodiment, the switch to a predetermined bit gain table isperformed to reduce the rate. During the period from time b to time c,when the training of the adjacent pair completes, the adjacent pairenters a power control phase and the crosstalk decreases, i.e., the linenoise decreases, so that the SNRM increases. Thus, according to theembodiment, the rate may be adjusted through the SRA to increase theline rate. During the period from time d to time e, when the subscriberof the adjacent pair turns off the modem, the crosstalk greatlydecreases, i.e., the line noise greatly decreases, so that the SNRMincreases. Thus, according to the embodiment, the rate may be adjustedthrough the SRA to increase the line rate.

To implement the embodiments, a switch message is also defined. In thestandard for VDSL2 (ITU-T G.993.2), an eoc (embedded overhead channel)message for an OLR command is defined. The command type field is(00000001)₂, subscript 2 represents binary, and other fields are definedin the following tables.

Name Length (octets) Octet number Content Request 5 + 4 × N_(f) 2 04₁₆(NOTE) Type 1 (N_(f) ≦ 128) 3 to 4 2 octets for the number ofsub-carriers N_(f) to be modified 5 to 4 + 4 × N_(f) 4 × N_(f) octetsdescribing the sub-carrier parameter field for each sub-carrier 5 + 4 ×N_(f) 1 octet for SC Request For further 2 05₁₆ (NOTE) Type 2 study Allothers Reserved by the ITU-T Request For further 2 06₁₆ (NOTE) Type 3study All others Reserved by the ITU-T (NOTE) - All other values foroctet number 2 are reserved by the ITU-T

TABLE 11-7/G.993.2 Reason codes for OLR responses Applicable ApplicableApplicable to to Reject to Reject Reason Octet value Defer Type 1 Type 2Type 3 Busy 01₁₆ X X X Invalid parameters 02₁₆ X X X

According to the above table, Type 1 has been used, and according to theembodiment, Type 2 or Type 3 may be used. The length of the fields isset to 2 octets, and the contents of the second octets are respectively(as shown in the table):05₁₆, 06₁₆, where subscript 16 represents hex.Another message Type 4 may be defined, whose length is 2 octets, thecontent of the second octet being 07₁₆. Therefore, the definitions inthe current standard are not affected. As such, the format of theresponse message corresponding to the switch message is as shown in thefollowing table.

Length Name (octets) Octet number Content Defer 3 2 81₁₆ (NOTE) Type 1 31 octet for reason code (Table 11-7) Request Reject 3 2 82₁₆ (NOTE) Type2 3 1 octet for reason code (Table 11-7) Request Reject 3 2 83₁₆ (NOTE)Type 3 3 1 octet for reason code (Table 11-7) Request IACK 3 2 8B₁₆(NOTE) 3 1 octet for SC (NOTE) - All other values for octet number 2 arereserved by the ITU-T

Therefore, if the existing Type 2 or Type 3 is directly adopted,corresponding message fields are modified. The length of a message forgranting the switch is 2 octets, where the second octet is 72₁₆ or 73₁₆(corresponding to the message type). The length of a message forrejecting the switch is 3 octets, where the second octet is 82₁₆ or 83₁₆(corresponding to the message type), and the third octet is the causefor rejecting (alternatively, if the cause is not needed, the length is2 octets). The length in case of newly defined Type 4, a response Type 4is defined and the length of a message for granting the switch is 2octets, where the second octet is 73₁₆, and the length of a message forrejecting the switch is 3 octets, where the second octet is 83₁₆, andthe third octet is the cause for rejecting (alternatively, if the causeis not needed, the length is 2 octets). The synchronization signal isbased on the signal defined in G.993.2, i.e., the synchronization signalis represented by switches between all-0 and all-1 of synchronizationsymbols.

In the above method, the messages have more capacity, and thus are ableto facilitate the communication between two parties. To increase theswitch speed, a switch message Type 2 or Type 3 or Type 4 may betransmitted without acknowledgement, and then the transceiver executesthe switch action upon receiving this message, and instructs thereceiving end through synchronization symbols to execute the synchronousswitch. If rejected, the synchronization message is not transmitted.Thus, the transmission and parsing process of the message content isavoided, thereby increasing the switch speed and reducing the errorprobability.

According to the above embodiment, two sets of parameters are calculatedin advance in the training, where one set is a bit table and gain tableobtained according to the current channel parameter (for example,channel noise), and another set is a conservative bit table and gaintable obtained according to the virtual noise or experiential bit tablecalculating parameter. These two sets of parameters are saved in thefirst transceiver and the second transceiver at the same time. When thetraining is completed, the transceiver uses the bit table and the gaintable obtained according to the current channel parameter, calledcurrent bit table and gain table, and this bit table and gain table isdynamically adjusted with the change in the channel (for example, bitswapping). When a very large wide band noise (for example, crosstalk)presents, a fast switch from the current bit table and gain table to thebit table and gain table corresponding to the virtual noise is performedby using a simple message. Because it is not necessary to exchange thebit table and gain table and only a simple message is exchanged,compared with the bit exchange, the solutions according to theembodiments have the advantages of fast switch speed and highreliability. When the above wideband noise decreases (for example, theuser of a crosstalk source turns off the modem), the SNRM may becalculated according to the current channel parameter, and the frequencyspectrum utilization may be increased by increasing the rate through theSRA.

In addition, because the switch speed in the solutions according to theembodiment is fast, link drops may be avoided by responding to thefeature of sudden increasing of the crosstalk. In case of SRA, failuresare likely to occur because of response speed and errors in theparameter exchange process, thereby causing the re-training andinterrupting the service.

To obtain the communication performance parameter, the followingquery-response mode may be adopted. Specifically, the line quality isdetermined according to the result of query-response. The method ofobtaining the line quality through the query-response mode is describedas follows.

As shown in FIG. 4, a query message and a response message areconstructed. FIG. 4( a) shows the query message, and FIG. 4( b) showsthe response message. It is assumed that the first transceiver transmitsthe query message to the second transceiver.

When the counter in the query message received by the second transceiveris X (X falls within the range of 0-255), the message is respondedimmediately, and the counter octet in the responded response messageshould be X+1. If an overflow occurs, the value of the counter octet isset to zero. When the first transceiver receives the response messageand the content of the counter octet is valid, a query message istransmitted after a time interval (T), and the content of the counteroctet is that of the valid counter octet received from the secondtransceiver. The further course is similar to the above. FIG. 5 shows aquery-response process between the first transceiver and the secondtransceiver.

As shown in FIG. 6, when the second transceiver does not correctlyreceive the query message from the first transceiver, the secondtransceiver does not respond. When the first transceiver does notcorrectly receive the response message from the receiving end, the firsttransceiver will continually re-transmit the query message identical tothe previous one at a prescribed time point.

To record the result of query-response, a register may be provided inthe first transceiver and the second transceiver, the bits of whichrespectively record receiving status of n consecutive messages. Tofacilitate the description, the registers in the first transceiver andthe first transceiver are represented respectively with TM and RM. Afterinitialization or after a bit table entry switch, the initial values ofthe TM and the RM should be zero.

For the first transceiver, within the current time frame, the firsttransceiver writes ‘1’ in the current bit of the TM if no valid responsemessage is received within the prescribed time interval, and writes ‘0’if a valid response message is received within the prescribed timeinterval. Then, the first transceiver receives the response messagewithin the next time frame, determines the next bit of the TM as thecurrent bit, and writes ‘1’ in the current bit of the TM if no validresponse message is received within the prescribed time interval, andwrites ‘0’ if a valid response message is received within the prescribedtime interval.

For the second transceiver, within the current time frame, the secondtransceiver writes ‘1’ in the current bit of the RM if no valid querymessage is received within the prescribed time interval, and writes ‘0’if a valid query message is received within the prescribed timeinterval; then, the second transceiver receives the query message withinthe next time frame, determines the next bit of the RM as the currentbit, and writes ‘1’ in the current bit of the RM if no valid querymessage is received within the prescribed time interval, and writes ‘0’if a valid query message is received within the prescribed timeinterval.

In the first transceiver, when a predetermined number (i.e., switchthreshold) of bits in the TM register are set to ‘1’, a decision is madeto perform the switch at a time agreed on between the first transceiverand the second transceiver, for example, to perform the switch of bittable and gain table at the tenth symbol of the next synchronizationframe. To ensure the synchronous switch, keep transmitting invalid dataafter the switch is decided, so that the second transceiver reaches theswitch condition as soon as possible, and thus the switch of bit tableand gain table is performed at the tenth symbol of the nextsynchronization frame. Thus, without the need to transmit the switchmessage between the first transceiver and the second transceiver, thesynchronous switch of bit table may be implemented, to cope with variousproblems such as link drop caused by sudden increase of the line noise(for example, crosstalk). The same mechanism is adapted for the secondtransceiver in order to perform the switch of bit table.

FIG. 7 shows a case of query-response where the TM and the RM have 5bits and the switch threshold is 3. As shown in FIG. 7, the solid linerepresents that the present message transmission is correct, and thedashed line represents that the present message transmission is wrong.In the first time frame, the first transceiver transmits a query messagehaving a counter octet of 001 to the second transceiver, and uponreceiving the query message; the second transceiver returns a responsemessage having a counter octet of 002 to the first transceiver. At thistime, the bits in the TM and the RM of the first transceiver and thesecond transceiver do not change, i.e., all are 0. In the second timeframe, the TM and the RM respectively shift by one bit circularly in aforward direction, the first transceiver transmits a query messagehaving a counter octet of 002 to the second transceiver, and notreceiving the query message, the second transceiver still returns aresponse message having a counter octet of 002 to the first transceiver.At this time, the current bit (the last one bit as shown in the figure)in the TM and the RM of the first transceiver and the second transceiverare all set to 1. In the third time frame, the TM and the RMrespectively shift by one bit circularly in a forward direction, thefirst transceiver transmits a query message having a counter octet of002 to the second transceiver, and upon receiving the query message, thesecond transceiver's receiving is correct, the current bit in the RM ofthe second transceiver keeps to 0, and then the second transceiverreturns a response message having a counter octet of 003 to the firsttransceiver. Because the first transceiver does not receive the responsemessage within the prescribed time interval, the current bit in the TMof the first transceiver is set to 1. Similar to these, when reaching tothe 7th time frame, the number of ‘1's (switch threshold) in the TM ofthe first transceiver reaches to 3, and from the 8th time frame, thefirst transceiver transmit a query message having a wrong counter octetto the second transceiver, so that the number of ‘1's in the RM of thesecond transceiver reaches to the switch threshold as soon as possible,in order to perform the switch of bit table at the tenth symbol of thenext synchronization frame. Because it is not necessary to re-transmitthe message between the first transceiver and the second transceiver,the synchronous switch of bit table can be implemented, and thesolutions according to the embodiments have the advantages of fastswitch speed and high reliability. When the above wideband noisedecreases (for example, the user of a crosstalk source turns off themodem), the SNRM may be calculated according to the current channelparameter, and the frequency spectrum utilization can be increased byincreasing the rate through the SRA, thus increasing the transmissionspeed.

When performing the switch of bit table, the switch of gain table may ormay not be performed.

In summary, a bit table and gain table are determined in advance, andthe determined bit table and gain table are respectively saved in thefirst transceiver and the second transceiver. When a very large wideband noise (for example, crosstalk) presents, a fast switch from thecurrent bit table and gain table to the previously determined bit tableand gain table is performed by using a simple message or aquery-response mechanism. Because it is not necessary to exchange thebit table and gain table between the first transceiver and the secondtransceiver, the solutions according to the embodiments have theadvantages of fast switch speed and high reliability. When the abovewideband noise decreases (for example, the user of a crosstalk sourceturns off the modem), the SNRM may be calculated according to thecurrent channel parameter, and the frequency spectrum utilization may beincreased by increasing the rate through the SRA.

Although the present invention has been described in connection with theembodiments, those skilled in the art know that, there can be manyvariations and changes to the present invention without departing fromthe spirit and substance of the present invention, and the scope of thepresent invention is defined by the appended claims.

1. A communication method for a digital subscriber line (DSL) systemcomprising a first transceiver and a second transceiver coupled to thefirst transceiver via a subscriber line, comprising: performing acommunication between the first transceiver and the second transceiverwith first bit numbers for sub-carriers, wherein the first bit numbersindicate a first bit allocation on the sub-carriers; transmitting aswitch message for requesting a rate decrease from the first transceiverto the second transceiver when the first transceiver determines that anobtained communication performance parameter reaches or exceeds thepredetermined value; and executing a switch to second bit numbers forthe sub-carriers at a certain symbol of a frame in response to thetransmission of the switch message, wherein the second bit numbersindicate a second bit allocation on the sub-carriers; wherein the firsttransceiver calculates the second bit numbers for the sub-carriers bysubtracting a corresponding bit number from the first bit numbers forthe sub-carriers; and wherein the first transceiver transmits thecorresponding bit number to the second transceiver for calculating thesame bit numbers as the second bit numbers.
 2. The method according toclaim 1, further comprising: performing a rate adjustment process forincreasing rate between the first transceiver and the second transceiverthrough a seamless rate adaptation (SRA) solution after executing theswitch,.
 3. The method according to claim 1, wherein the second bitnumbers for the sub-carriers start to be used from n^(th) symbol in asynchronization frame, n indicating a symbol count.
 4. The methodaccording to claim 1, wherein the communication performance parametercomprises at least one of a signal to noise ratio, a signal to noiseratio margin, and a bit error rate.
 5. The method according to claim 1,wherein the second bit numbers for the sub-carriers are calculated afterthe communication performance parameter is determined to reach or exceedthe predetermined value.
 6. The method according to claim 1, wherein thesecond bit numbers for the sub-carriers are calculated and stored at thefirst transceiver before the communication performance parameter isdetermined to reach or exceed the predetermined value.
 7. A transceiverin a digital subscriber line (DSL) system, comprising: a receivingmodule configured to receive a signal from an opposite transceiver overa subscriber line; a monitor module coupled to the receiving module andconfigured to obtain a communication performance parameter; a processingmodule coupled to the monitor module and configured to calculate firstbit numbers for sub-carriers in the DSL system, and generate a switchmessage for requesting a rate decrease when the communicationperformance parameter reaches or exceeds a predetermined value, andexecute a switch to the first bit numbers for the sub-carriers from acertain symbol of a frame, wherein the first bit numbers for thesub-carriers indicate a first bit allocation on the sub-carriers that isconfigured to achieve communication between the transceiver and theopposite transceiver after switching, wherein the transceiver calculatesthe first bit numbers for the sub-carriers by subtracting acorresponding bit number from second bit numbers for the sub-carriers;wherein the second bit numbers indicate a second bit allocation on thesub-carriers that is configured to achieve communication between thetransceiver and the opposite transceiver before switching; wherein thecorresponding bit number is transmitted by the transceiver to theopposite transceiver for calculating the same bit numbers for thesub-carriers as the first bit numbers for the sub-carriers; and atransmitting module coupled to the processing module and configured totransmit the switch message through the subscriber line to the oppositetransceiver.
 8. The transceiver according to claim 7, wherein thetransceiver comprises: a seamless rate adaptor (SRA) configured tocalculate third bit numbers indicating a third bit allocation forsub-carriers according to an actual noise, and take the third bitnumbers indicating the third bit allocation for sub-carriers to increaserate after switching to the first bit numbers for rate decrease.
 9. Thetransceiver according to claim 7, wherein the certain symbol is n^(th)symbol in a synchronization frame, n indicating a symbol count.
 10. Thetransceiver according to claim 7, wherein the communication performanceparameter comprises at least one of a signal to noise ratio, a signal tonoise ratio margin, and a bit error rate.
 11. The transceiver accordingto claim 7, wherein the first bit numbers for the sub-carriers arecalculated after the communication performance parameter is determinedto reach or exceed the predetermined value.
 12. The transceiveraccording to claim 7, wherein the first bit numbers for the sub-carriersare calculated and stored at the transceiver before the communicationperformance parameter is determined to reach or exceed the predeterminedvalue.
 13. A communication method for a digital subscriber line (DSL)system comprising a first transceiver and a second transceiver coupledto the first transceiver via a subscriber line, comprising: performing acommunication between the first transceiver and the second transceiverwith first bit numbers for sub-carriers, wherein the first bit numbersindicate a first bit allocation on the sub-carriers; receiving at thefirst transceiver a switch message for requesting a rate decrease fromthe second transceiver; and after the reception of the switch message,executing at the first transceiver a switch to second bit numbers forthe sub-carriers at a certain symbol of a frame to implementcommunication with the second transceiver, wherein the second bitnumbers indicate a second bit allocation on the sub-carriers; whereinthe first transceiver receives the corresponding bit number from thesecond transceiver and calculates the second bit numbers for thesub-carriers by subtracting the corresponding bit number from the firstbit numbers for the sub-carriers.
 14. The method according to claim 13,further comprising: performing a rate adjustment process for increasingrate between the first transceiver and the second transceiver through aseamless rate adaptation (SRA) solution after executing the switch. 15.The method according to claim 13, wherein the certain symbol is n^(th)symbol in a synchronization frame, n indicating a symbol count.
 16. Themethod according to claim 13, wherein the communication performanceparameter comprises at least one of a signal to noise ratio, a signal tonoise ratio margin, and a bit error rate.
 17. A transceiver in a digitalsubscriber line (DSL) system, comprising: a transmitting moduleconfigured to transmit a signal to an opposite transceiver coupledthrough a subscriber line to the transceiver; a receiving moduleconfigured to receive from the opposite transceiver a switch message forrequesting a rate decrease; and a processing module configured tocalculate first bit numbers for sub-carriers by subtracting acorresponding bit number from second bit numbers for the sub-carriers;and execute a switch to the first bit numbers for the sub-carriers at acertain symbol of a frame; wherein the first bit numbers indicate afirst bit allocation on the sub-carriers; where the second bit numbersfor the sub-carriers indicate a second bit allocation on thesub-carriers and are configured within the transceiver to implementcommunication with the opposite transceiver, wherein the correspondingbit number is received by the transceiver via the receiving module fromthe opposite transceiver for calculating the first bit numbers for thesub-carriers.
 18. The transceiver according to claim 17, wherein thetransceiver comprises: a seamless rate adaptor (SRA) configured tocalculate third bit numbers for sub-carriers according to an actualnoise, and take the third bit numbers for sub-carriers to increase rateafter the switch to the bit numbers.
 19. The transceiver according toclaim 17, wherein the certain symbol is n^(th) symbol in asynchronization frame, n indicating a symbol count.
 20. The transceiveraccording to claim 17, wherein the communication performance parametercomprises at least one of a signal to noise ratio, a signal to noiseratio margin, and a bit error rate.